There is a need for lignocellulosic composites that are dimensionally stable when exposed to moisture. There is a further need for lignocellulosic composites that do not swell when immersed in water and that do not shrink when dried. There is still a further need for lignocellulosic composites that do not darken in color when formed in a hot press.
Lignocellulosic composites are conventionally manufactured by hot pressing lignocellulosic materials with wax and thermosetting resin. This is referred to as a conventional bonding process. The wax is a sizing agent to improve the water resistance of the once-formed composite. The resin is a binding agent that holds the materials comprising the composite together, thus forming them into a unitary shape. Resoles are commonly used as the binding resin for lignocellulosic composites. The use of resoles to bind the composite results in a darkening of the color of the composite on setting of the resin in the hot press.
In the conventional hot press method of manufacture of lignocellulosic composites, a lignocellulosic material is combined with a phenolic resin and other components in a blender or mixer. The blend or mixture that results is pressed, typically under pressures above atmospheric and temperatures greater than room temperature, to produce the composite. Lignocellulosic materials used in the production of mats may be selected from the group consisting of wood fiber, wood flake, wood strands, wood chips and wood particles, and mixtures thereof. The lignocellulosic materials listed here are referred to in the art as wood furnish. However, it is well known that other wood furnish, such as straw, bagasse, wood bark, recycled wood fiber, recycled paper fiber, and mixtures thereof, may also be used. The wood furnish, once blended or mixed with the phenolic resin, is then formed onto a support material to make a pre-form in the approximate shape of the finished good. The pre-form is then placed on a caul plate in a hot press where the finished good is produced by applying pressures above atmospheric and temperatures greater than room temperature. The elevated temperatures and pressures cause the phenolic resin to polymerize, thus binding the pre-form into a unitary finished good. The hot press method is further described in U.S. Pat. No. 4,433,120 to Shui-Tung Chiu.
Lignocellulosic composites primarily find use in construction or fabrication. These composites may be used in building construction or any fabrication where wood is a traditional material used. The poor dimensional stability of state-of-the-art lignocellulosic composites affects their mechanical properties and reduces their load carrying ability. Another result of poor dimensional stability is unevenness of roof and floor underlayments, and of building siding.
Two methods have been principally suggested as means to produce dimensionally stable lignocellulosic composites. However, both of these method have proven to be too costly to be used in practice. The first method is referred to as Bulking Treatment. In this method, lignocellulosic materials are impregnated with water soluble polymers such as polyethylene glycol or impregnated with a low molecular weight resin such as phenol-formaldehyde or vinyl monomers and polymerized in situ. The second method is referred to as Chemical Modification. In this method the lignocellulose may be esterified by, for example, acetylation, or it may be cross-linked using, for example, an aldehyde. An alternative method of Chemical Modification is to react hemicellulose with lignin under elevated temperatures, typically using steam treatment. Any of these methods of Chemical Modification, in addition to being costly, also result in reduced strength of the once-formed composite.
A method widely used in the conventional bonding process to improve dimensional stability, as noted above, is the application of a wax sizing agent. This wax sizing imparts a certain degree of water repellency to the once-formed composite. Paraffin is a common sizing agent. One method by which wax sizing impart water repellency is by coating the surface of the lignocellulose, thus decreasing its surface tension. Another method by which wax sizing imparts water repellency is that the wax will partially fill the capillaries within the lignocellulose, thus providing a barrier to the capillary uptake of water.
Hydrogen peroxide has been used in conventional bonding processes. Chapman and Jenkin, in "Hydrogen Peroxide as a Resin Cure Accelerator," Journal of Adhesion, Volume 19 (1986), disclose the use of hydrogen peroxide for its heating potential. The exothermic reaction of hydrogen peroxide may be used to reduce the hot pressing time for making medium density fiberboard and particleboard with urea-formaldehyde and tannin resin. However, Chapman and Jenkin did not use wax in their formulation and did not evaluate the water resistance of their products. Chapman and Jenkin, in this same reference, described the treating of a veneer surface with hydrogen peroxide for the reduction of the press time of a plywood made with a phenol- formaldehyde resin and tannin resin.
There are also known non-conventional bonding processes for manufacturing lignocellulosic composites. These methods are characterized by surface treatment of the lignocellulose with an oxidant and then either hot pressing the treated lignocellulose to form the composite, with or without a catalyst, or applying a polymeric just before hot pressing. Examples of these non-conventional bonding processes follow.
Zavarin has shown that hydrogen peroxide may be used as the oxidant in non-conventional bonding processes. See E. Zavarin, "Activation of Wood Surfaces and Non-Conventional Bonding," The Chemistry of Solids Wood, ACS Advances in Chemistry Series No. 207 (1984). The hydrogen peroxide is believed to induce surface activation for bonding in which a conventional resin binder is not used.
Another non-conventional bonding process is described in U.S. Pat. No. 4,022,965 to Goheen, et al. In the process disclosed therein, hydrogen peroxide, in the presence of a sulfuric acid catalyst, is used to first oxidize wood chip fibers. Then the wood chips are mechanically refined to produce fibers. Next, the residual oxidant and catalyst are removed by water washing before the fibers are hot pressed to form the composite.
Yet another non-conventional bonding process is described in U.S. Pat. No. 4,007,321 to Scholz, et al. Here the inventors disclose a means to bond veneers or wood particles comprising applying hydrogen peroxide, among other oxidants, a metal-acid catalyst, and then hot pressing the mixture. By this process, a pH of from 0.5 to 1.5 is required at hot pressing in order to produce a weather resistant composite.
One non-conventional bonding process, disclosed by Philippou et al. in "Bonding of Particleboard Using Hydrogen Peroxide," Forest Products Journal, Volume 32 (1982), is said to produce water-resistant flakeboards. Here the authors disclose a process wherein wood flake is first treated with hydrogen peroxide. The wood flake is next mixed with ammonium lignosulfonate, furfuryl alcohol, and maleic acid, which then react to form the composite. Maleic acid is a catalyst for the cross-linking reaction of the ammonium lignosulfunate and furfuryl alcohol. Treating the wood apparently provides grafting sites for the polymers from the reaction of the ammonium lignosulfunate and furfuryl alcohol. The water resistance of the resulting flakeboard can be improved by incorporating a wax emulsion with the addition of the reactants. However, it is reported that the use of the wax emulsion negatively effects the mechanical properties of the flakeboard when it is applied at 1% or higher based on the dry wood weight.
Philippou et al. in "Bonding Wood by Graft Polymerization. The Effect of Hydrogen Peroxide Concentration on the Bonding and Properties of Particleboards." Holzforschung, Volume 36 (1982), also disclose activating wood particle surfaces by applying hydrogen peroxide and then bonding the wood particles using acid-catalyzed polymerizable materials. Here, as previously described, cross-linking of mixtures of ammonium lignosulfonate, and furfuryl alcohol or formaldehyde, with lignocellulose is catalyzed using maleic acid. However, the authors disclose no use of wax or wax emulsions. Resistance to water or moisture, measured by the resistance to swelling upon exposure, was reported to be improved by use of increasing amounts of hydrogen peroxide.
The phenol-formaldehyde resin used in the manufacture of lignocellulosic composites may be in the form of a solid or a liquid. Powdered phenolic resins, such as novolac, resole, or combinations thereof, may generally be used. U.S. Pat. No. 4,098,770 to Berchem, et al., discloses a typical spray-dried phenol-formaldehyde resin, modified with added non-phenolic polyhydroxy compounds, used in the manufacture of waferboard. Liquid phenol-formaldehyde resins, such as resole or resole and novolac combinations, may also be generally used in the manufacture of lignocellulosic composites. Parameters for the manufacture of either liquid or solid phenol-formaldehyde resins are disclosed in Phenolic Resins, Chemistry, Applications and Performance, (A. Knop and L. A. Pilato, Springer-Verlag (1985)) and Advance Wood Adhesives Technology, (A Pizzi, Marcel Dekker (1994)).